FLOW-INJECTION SPECTROPHOTOMETRIC DETERMINATION OF FREE AND TOTAL SULFITE IN WINES BASED ON THE INDUCED OXIDATION OF MANGANESE(II)

The sulfur(IV) content in wines was determined with a flow injection s pectrophotometric system by treating each sample in the donor channel with sulfuric acid in order to release free SO2, which diffuses throug h a PTFE membrane to the acceptor solution containing Mn(II) in acetat e buffer at pH 5.5. The induced oxidation of Mn(II) takes place propor tionally to the free S(IV) content and the oxidized species formed rea cts with iodide to form iodine which can be spectrophotometerically me asured at 352 nm. A linear plot of absorbance vs S(IV) concentration i s observed up to 26 mg SO2 L-1 with a detection limit of 1.0 mg L-1. M atrix effects required the standard additions of sulfite to the sample s for obtaining accurate results. A previous treatment of the sample w ith tris(hydroxymethylaminomethane) and EDTA decomposed the hydrogen-s ulfite/aldehyde adducts and allowed the determination of the total S(I V) content. As expected, total S(IV) concentration in wines has been f ound to be much higher than free sulfite. A recovery-addition experime nt with red and white wines led to recoveries varying from 91.5 to 109 .7% by applying the free and total sulfite developed procedures. This is a good indication of the accuracy of both proposed procedures.